GaN (gallium nitride) is a nitride semiconductor material whose bandgap is larger than that of Si (silicon), and has excellent material properties such as a high saturated electron speed and a high breakdown electric field intensity. A GaN layer can form a hetero junction with a group III-V compound semiconductor layer containing an element such as Al (aluminum) or In (indium). At the hetero interface between the GaN layer and an AlGaN (aluminum gallium nitride) layer, an electron layer having a high electron concentration and a high electron mobility called two-dimensional electron gas (2 DEG) layer is generated by spontaneous polarization or piezo polarization. A high electron mobility transistor (HEMT) using such properties of the hetero junction has been attracting attention as a next-generation device to be used in a power amplifier and a switching device.
An example of requirements on the HEMT is, for example, to have high performance such as a high dielectric breakdown voltage and a high current operation. The HEMT is also required to have a reduced chip size for lowering a chip cost, and to have a wafer composition suitable for carrying out an efficient conduction test such as burn-in in a wafer state.
The GaN layer and the AlGaN layer of the HEMT are stacked on a semiconductor substrate via a buffer layer. However, the semiconductor substrate and the GaN layer are different from each other in lattice constant and thermal expansion coefficient. Therefore, the buffer layer, the GaN layer and the AlGaN layer include dislocations which are a kind of crystal defects. These dislocations cause a leakage current when a high voltage is applied to the HEMT. Furthermore, when the breakdown voltage of the HEMT is increased, carriers are generated between the semiconductor substrate and the buffer layer to form an inversion layer or an accumulation layer (which is also referred to as a conductive layer). As a result, a state like a short channel is generated between the source electrode and the drain electrode, thereby generating a leakage current. These leakage currents serve as barriers against improvements of the breakdown voltage of the HEMT.
On the other hand, a bonding pad and a bonding wire to be electrically connected to a circuit terminal of the HEMT is typically placed on the outside (periphery) of a semiconductor chip including the HEMT. If the area of the bonding pad is reduced in this configuration, the bonding position of the bonding wire may be deviated in a wire bonding process. This results in a degraded bonding strength, a contact failure of the bonding wire, an increase in resistance of the bonding wire, and the like. This makes it difficult to reduce the chip area.